you need to get an smd importer/exporter plugin for your 3d modelling aplication, and export your model as an smd file and write out a qc script that you use to compile the smd's into the correct format using the compiler provided as a source engine tool with all the games. a quick google search should get you a quick export method for static objects, character models are a bit more of an arduous process, that also include shape keys for facial expressions.